(A) Field of the Invention
This invention relates to a method for treating (regenerating and reactivating, or activating) Group VIII noble metal-containing coke-contaminated catalysts and fresh catalysts commonly used in hydrocarbon conversion processes, especially catalytic reforming. The catalysts which can be treated according to the present invention will contain a support, more preferably, the catalysts will contain both a support and a binder.
(B) Description of the Prior Art
Several materials have been employed as hydrocarbon conversion catalysts in processes such as reforming, catalytic dewaxing, alkylation, oxidation and hydrocracking. Examples of catalysts useful for these processes include materials consisting of a catalytically active Group VIII metal, typically platinum, and optionally rhenium or tin, carried on or impregnated into a support. Conventionally this support is alumina although more recently zeolites have been employed, for example type X and Y zeolites. Type L zeolites have also been suggested as suitable supports, especially cation exchanged type L zeolites.
Among the hydrocarbon conversion processes, catalytic reforming in the presence of hydrogen is one of the most important. Catalytic reforming is a refinery process designed to increase the octane number of naphtha. Typically the naphtha feed is passed over a platinum-containing catalyst supported on a refractory material under reforming conditions, for example, elevated temperatures and pressures, well-known in the industry in the presence of hydrogen gas with a hydrogen to hydrocarbon mole ratio of about 2 to 20. The reforming process involves several different types of reactions including isomerization, dehydrogenation of naphthenes to aromaticks, dehydrogenation of paraffins to olefins, dehydrocyclization of paraffins and olefins to aromatics, hydrocracking of paraffins to gaseous hydrocarbons such as methane or ethane, and inevitably the formation of coke, the latter being deposited on the catalyst. Ideally the reforming process minimizes the hydrocracking of parafins and maximizes the reactions leading to the formation of more valuable products, particularly dehydrocyclization and dehydrogenation to aromatics.
During the reforming process the activity of the catalyst gradually declines due to the build-up of coke, until eventually it requires regenerating and reactivating. There are several steps required for the regeneration and reactivation of the catalyst.
The first step is the regeneration of the catalyst by removing the deposited coke. Typically the coke is removed by heating the catalyst in the presence of dilute oxygen at a flame-front temperature of 430.degree. C. to 540.degree. C. Some water vapor may be present in this regeneration step. U.S. Pat. No. 4,354,925 discloses using a gaseous mixture of oxygen and carbon dioxide, the presence of this gas mixture permitting a higher oxygen concentration to be used in the burn gas, thereby enabling the coke burn off to be completed more quickly. This regeneration step may be preceded by flushing the system with, for example, hydrogen or nitrogen gas to remove residual hydrocarbons.
The high temperatures used in the above regeneration lead to agglomeration of the metal particles and, when the support is a zeolite, to removal of the metal particles from the zeolite channels, and hence to deactivation of the catalyst. Thus, after the coke burn off, the catalyst is usually subjected to a reactivation step in which the metal particles are redispersed on the support material. Typically this involves treating the catalyst with chlorine-containing gas or a chlorine-liberating gas usually in the presence of oxygen and water vapor, and is often referred to as the oxychlorination step.
Subsequently the catalyst is treated with a reducing gas, typically hydrogen, to reduce the metal chloride formed in the above oxychlorination step to elemental metal particles. The catalyst is then ready for reuse in the reforming process.
However, it has been found that, particularly platinum-containing zeolite catalysts, the above regeneration and reactivation process often does not reactivate the catalyst to activation levels at or near those exhibited by the fresh catalyst.